The present invention relates to semiconductor chip assemblies and to methods and components useful in making such assemblies.
Complex microelectronic devices such as modern semiconductor chips require numerous connections to other electronic components. For example, a complex microprocessor chip may require many hundreds of connections to external devices.
Semiconductor chips commonly have been connected to electrical traces on mounting substrates by one of three methods: wire bonding, tape automated bonding, and flip-chip bonding. In wire bonding, the semiconductor chip is positioned on a substrate with a bottom or back surface of the chip abutting the substrate and with the contact-bearing front or top surface of the chip facing upwardly, away from the substrate. Individual gold or aluminum wires are connected between the contacts on the semiconductor chip and current conducting pads on the substrate. In tape automated bonding a flexible dielectric tape with a prefabricated array of leads thereon is positioned over the semiconductor chip and substrate, and the individual leads are bonded to the contacts on the chip and to the current conducting pads on the substrate. In both wire bonding and conventional tape automated bonding, the current conducting pads on the substrate are arranged outside of the area covered by the semiconductor chip, so that the wires or leads fan out from the chip to the surrounding current conducting pads. The area covered by the subassembly as a whole is considerably larger than the area covered by the chip. This makes the entire assembly substantially larger than it otherwise would be. Because the speed with which a microelectronic assembly can operate is inversely related to its size, this presents a serious drawback. Moreover, the wire bonding and tape automated bonding approaches are generally most workable with semiconductor chips having contacts disposed in rows extending along the periphery of the chip. They generally do not lend themselves to use with chips having contacts disposed in a so-called area array; i.e., a grid-like pattern covering all or a substantial portion of the chip front surface.
In the flip-chip mounting technique, the contact bearing surface of the semiconductor chip faces towards the substrate. Each contact on the semiconductor chip is joined by a solder bond to the corresponding current carrying pad on the substrate, as by positioning solder balls on the substrate or contacts of the semiconductor chip, juxtaposing the chip with the substrate in the front-face-down orientation and momentarily melting or reflowing the solder. The flip-chip technique yields a compact assembly, which occupies an area of the substrate no larger than the area of the chip itself. However, flip-chip assemblies suffer from significant problems with thermal stress. The solder bonds between the contacts on the semiconductor chip and the current carrying pads on the substrate are substantially rigid. Changes in the size of the chip and of the substrate due to thermal expansion and contraction in service create substantial stresses in these rigid bonds, which in turn can lead to fatigue failure of the bonds. Moreover, it is difficult to test the semiconductor chip before attaching it to the substrate, and hence difficult to maintain the required outgoing quality level in the finished assembly, particularly where the assembly includes numerous semiconductor chips.
Numerous attempts have been made to solve the foregoing problem. Useful solutions are disclosed in commonly assigned U.S. Pat. Nos. 5,148,265 and 5,148,266. Preferred embodiments of the structures disclosed in these patents incorporate flexible, sheet-like structures referred to as xe2x80x9cinterposersxe2x80x9d or xe2x80x9cchip carriers.xe2x80x9d The preferred chip carriers have a plurality of terminals disposed on a flexible, sheet-like top layer. In use, the interposer is disposed on the front or contact bearing surface of the chip with the terminals facing upwardly, away from the chip. The terminals are then connected to the contacts of the chip. Most preferably, this connection is made by bonding prefabricated leads on the interposer to the contacts on the semiconductor chip, using a tool engaged with the lead. The completed assembly is then connected to a substrate, as by bonding the terminals of the chip carrier to the substrate. Because the leads and the dielectric layer of the chip carrier are flexible, the terminals on the chip carrier can move relative to the contacts on the semiconductor chip without imposing significant stresses on the bonds between the leads and the contacts on the semiconductor chip, or on the bonds between the terminals of the chip carrier and the substrate. Thus, the assembly can compensate for thermal effects. Moreover, the assembly most preferably includes a compliant layer disposed between the terminals on the chip carrier and the face of the semiconductor chip itself as, for example, an elastomeric layer incorporated in the chip carrier and disposed between the dielectric layer of the chip carrier and the semiconductor chip. Such a compliant structure permits displacement of the individual terminals independently towards the chip, and also facilitates movement of the terminals relative to the chip in directions parallel to the chip surface. The compliant structure further enhances the resistance of the assembly to thermal stresses during use, and facilitates engagement between the subassembly and a test fixture during manufacture. Thus, a test fixture incorporating numerous electrical contacts can be engaged with all of the terminals in the subassembly despite minor variations in the height of the terminals. The subassembly can be tested before it is bonded to a substrate so as to provide a tested, known, good part to the substrate assembly operation. This, in turn, provides very substantial economic and quality advantages.
U.S. Pat. No. 5,455,390, the disclosure of which is hereby incorporated in its entirety herein, describes a further improvement. Components according to preferred embodiments of the ""390 patent use a flexible, dielectric top sheet. A plurality of terminals are mounted on the top sheet. A support layer is disposed underneath the top sheet, the support layer having a bottom surface remote from the top sheet. A plurality of electrically conductive, elongated leads are connected to the terminals on the top sheet and extend generally side by side downwardly from the terminals through the support layer. Each lead has a lower end at the bottom surface of the support layer. The lower ends of the leads have conductive bonding materials such as, for example, eutectic bonding metals. The support layer surrounds and supports the leads.
Components of this type can be connected to microelectronic elements such as semiconductor chips or wafers by juxtaposing the bottom surface of the support layer with the contact-bearing surface of the semiconductor chip so as to bring the lower ends of the leads into engagement with the contacts on the chip, and then subjecting the assembly to elevated temperature and pressure conditions. All of the lower ends of the leads bond to the contacts on the semiconductor chip substantially simultaneously. The bonded leads connect the terminals of the top sheet with the contacts on the semiconductor chip. The support layer desirably is either formed from a relatively low-modulus, compliant material, or else is removed and replaced after the lead bonding step with such a compliant material. In the finished assembly, the terminals on the relatively flexible dielectric top sheet desirably are movable with respect to the contacts on the semiconductor chip to permit testing and to compensate for thermal effects. However, the components and methods of the ""390 patent provide further advantages, including the ability to make all of the bonds to the chip or other component in a single lamination-like process step.
U.S. Pat. No. 5,518,964, the disclosure of which is hereby incorporated by reference herein, discloses still further improvements. Preferred methods according to the ""964 Patent include the steps of providing a dielectric connection component or first element having a plurality of terminals, a first surface and a plurality of elongated, flexible leads extending along the first surface, each such lead having a terminal end attached to one of the terminals and a tip end offset from the terminal end in a generally horizontal direction parallel to the first surface. Desirably, the tip ends of all the leads are attached to an associated one of the contacts on a second element, such as a microelectronic device. The preferred methods also include the step of simultaneously forming all of the leads by moving all of the tip ends of the leads relative to the terminal ends thereof and relative to the first element so as to bend the tip ends away from the first element. Desirably the step of moving the tip ends of the respective leads relative to their terminal ends includes the step of moving the second element relative to the first element. The first and second elements desirably move in a vertical direction, away from one another, and may also move in horizontal directions parallel to the surfaces of the elements so as to bend the tip end of each lead horizontally towards its own terminal end and vertically away from the terminal end. The net effect is to deform the leads towards formed positions in which the leads extend generally vertically downwardly, away from the first element. These methods may also include the step of injecting a flowable, desirably compliant dielectric material around the leads after the lead-forming step and then curing the flowable material so as to form a dielectric support layer around the leads.
In particularly preferred methods according to the ""964 Patent application, one element is a flexible, dielectric top sheet having terminal structures thereon, and the other element includes one or more semiconductor chips. The resulting assembly thus includes the dielectric top sheet with the terminal structures connected to the associated contacts of the semiconductor chip or chips by the vertically-extending, curved flexible leads, the dielectric top sheet being spaced apart from the semiconductor chip or chips by the dielectric support layer. The terminal structures can be connected to a substrate such as a circuit panel to thereby provide electrical current communication to the contacts on the semiconductor chip or chips. Each terminal structure on the dielectric top sheet is movable with respect to the contacts in the semiconductor chip in horizontal directions parallel to the chip, as well as in vertical directions towards and away from the chip, to take up differences in thermal expansion between the chip and substrate and to facilitate testing and assembly.
The step of attaching the tip ends of the leads to the second element desirably includes the step of bonding the tip ends of the leads to the contacts on the semiconductor chip or chips while the leads are in their initial, undeformed positions. For example, a dielectric sheet having the leads disposed in generally horizontal orientation on its bottom surface may be juxtaposed with a chip or wafer so that the tip ends of the leads are engaged with the contacts of the chip or wafer. Thus, all of the tip ends are bonded simultaneously to the chip contacts. A single simultaneous bonding operation may bond hundreds or thousands of leads. Because the leads are in their initial, undeformed positions when bonded to the contacts, the positions of the lead tips are well controlled at this stage. Preferably, the tip ends of the leads are releasably bonded to the dielectric top sheet. This facilitates registration of the lead tips contacts on the semiconductor chips. As the top sheet is moved upwardly away from the chip or wafer, the tip ends of the leads are released from the top sheet. The ""964 Patent also discloses other processes, including processes in which the leads are initially attached to the chip or wafer, and wherein the chip or wafer is engaged with dielectric sheet or other element having contacts thereon and the tip ends of the leads are bonded to the contacts. Although the teachings of the ""964 Patent can be applied in manufacture of many different products, one product taught in the ""964 Patent is a so-called xe2x80x9cchip size packagexe2x80x9d. The finished assembly, with the top sheet, terminals and compliant dielectric support layer, can be mounted within an area of a substrate substantially the same as that required to mount the chip itself.
A sheet-like elastomeric interposer having wires extending therethrough is disclosed in U.S. Pat. No. 5,531,022. The wires extend from a top surface to a bottom surface of the interposer. Balls on the ends of the wires are exposed at the top and bottom surfaces of the interposer for connecting to microelectronic elements. The wires may extend diagonally within the interposer and protuberances may project from the balls; those features are said to promote wiping between the balls and mating terminals.
Despite these and other advances in the art, there are still needs for further improvement.
One aspect of the present invention provides a component for forming connections to a microelectronic element. The component includes a compliant, sheet-like body having oppositely facing top and bottom surfaces. The top and bottom surfaces define outside surfaces of the connection component. As used herein, the term xe2x80x9csheet-likexe2x80x9d means having a thickness much smaller than a length or a width. Thus, the compliant, sheet-like body has a thickness between its top and bottom surfaces that is substantially smaller than the length and width of the body.
The component according to the invention has a plurality of electrically conductive, sheet-like first pads directly attached to the body at the top surface, and a plurality of electrically conductive, sheet-like second pads directly attached to the body at the bottom surface. A plurality of flexible, electrically conductive, elongated leads interconnect the first conductive pads with corresponding second conductive pads. The leads extend between the bonding pads through the compliant body and thereby electrically connect the corresponding first and second conductive pads. The compliant body surrounds and supports the leads.
The leads of the connection component of the invention may be bent or curved, and may include wires interconnecting the first and second bonding pads. The wires may be bonded to at least one of the first and second bonding pads with a conductive epoxy. The wires may be between about 100 and 1,000 microns long and between about 25 and 100 microns in diameter.
The compliant, sheet-like body may be an elastomeric layer. The bonding pads may be gold; the leads may also be gold. In the preferred embodiment of the invention, only one lead interconnects each set of corresponding first and second conductive pads.
The top surface of the compliant layer may include raised portions surrounding the first pads. The bottom surface may have a plurality of recesses corresponding in location to the location of the raised portions on the top surface. Further, the compliant layer may define a plurality of holes extending substantially from the top to the bottom surfaces. The holes extend in regions of the compliant layer between the leads. The raised portions, recesses and holes further increase the compliancy of the compliant layer by providing empty spaces into which the compliant layer may deform.
The connection component of the invention provides a compliant means for connecting microelectronic components. The compliance of the connector absorbs relative movement between the components caused by mechanical or thermal effects. Further, the compliance compensates for nonplanarity of mating terminal arrays by permitting individual contacts to move relative to each other. The component presents flat, sheet-like bonding pads on the surfaces of the compliant body, permitting the forming of high-quality bonds, and increasing the tolerance of the connection component to positional errors in the terminals of the mating microelectronic components.
A further aspect of the invention provides a method for making a microelectronic connector. A method according to this aspect of the invention includes providing a first base sheet having a first major surface with a plurality of first conductive terminal portions. The terminal portions of the base sheet may be integral portions of the base sheet itself, may be conductive pads formed on a surface of the base sheet, or may be combinations of both. A plurality of leads is formed on the base sheet, with first ends of the leads connected to corresponding ones of the first conductive terminal portions, and second ends of the leads remote from the corresponding first base sheet. A compliant layer is then formed adjacent the first major surface of the base sheet. The compliant layer surrounds the leads, and has a first major surface abutting the first major surface of the base sheet. Surplus portions of the first base sheet are then removed, leaving the first terminal portions affixed to the compliant layer at the first major surface of the compliant layer.
Desirably, the first conductive terminal portions of the base sheet are conductive pads on the first major surface of the base sheet. The conductive pads are formed from material having a greater resistance to a selected etchant than the base sheet. The step of removing a portion of the base sheet comprises etching the base sheet with the selected etchant so as to leave the conductive pads substantially intact on the first major surface of the compliant layer. In that case, the step of forming the leads may comprise connecting the first ends of the leads to the conductive pads.
The conductive first terminal portions may alternatively be integral portions of the base sheet. In that case, the step of removing surplus portions of the base sheet comprises selectively removing portions of the base sheet outside the terminal portions. In this embodiment, the terminal portions of the base sheet may further comprise conductive pads formed on a back surface opposite the first major surface of the base sheet.
Most preferably, the first ends of the leads are displaced with respect to the second ends of the leads, deforming the leads. By forming bends or curves in the leads, the leads are permitted to extend slightly in use, thereby allowing relative expansion, contraction and movement of two microelectronic elements joined by the connector. Deformation of the leads thus creates xe2x80x9cslackxe2x80x9d in the leads to permit later relative movement.
Desirably, the method of the invention further includes the steps of providing a second base sheet having a second major surface and including a plurality of second conductive terminal portions, and bonding the second ends of the leads to corresponding ones of the second terminal portions. The compliant layer is thus formed between the first and second base sheets, and the step of forming the compliant layer includes forming a second major surface of the compliant layer abutting the second major surface of the second base sheet. In this case, the removing step further comprises removing portions of the second base sheet, whereby the second terminal portions remain affixed to the compliant layer at the second major surface.
The above-described method may be performed using a lead forming tool to form the plurality of leads substantially simultaneously. The lead forming tool has a tool surface facing the first major surface of the first base sheet. After forming the leads on the first base sheet, the lead forming tool is separated from the first base sheet so that the lead wires pay out from the lead forming tool. The compliant layer is then formed between the tool and the base sheet so as to form a second major surface of the compliant layer abutting the tool surface of the lead forming tool.
The method may also include the step of forming asperities on the first terminal portions of the base sheet. The asperities aid in breaking through any oxide layers present on the mating terminals of a microelectronic element, or on the finished terminal portions of the connector of the invention. The asperities may be formed by forming an etch-resistant pattern on back surfaces of the terminal portions, so that a pattern including asperities is formed on the terminal portions as the base sheet is etched.
The method may additionally include the step of etching the first major surface of the base sheet in regions surrounding the first conductive terminal portions. The etching step forms recessed portions of the base sheet surrounding the terminal portions. In this embodiment, the step of forming the compliant layer forms raised portions of the compliant layer abutting the recessed portions of the base sheet and surrounding the first terminal portions. The method may further comprise the step of forming holes in the compliant layer extending substantially from the first major surface to the second major surface.
According to a further, and particularly preferred method of the invention, a microelectronic connector is made by providing a first sacrificial base sheet with a first major surface. The first major surface has a plurality of first conductive terminals disposed thereon. A plurality of leads extending away from the base sheet is then formed. The leads have first ends connected to corresponding ones of the first conductive terminals, and have second ends remote from those terminals.
A second sacrificial base sheet is then provided having a second major surface. The second base sheet has a plurality of second conductive terminals disposed on the second major surface. The second ends of the leads are bonded to corresponding ones of the second terminals, electrically connecting the first and second conductive terminals through the leads. A compliant material is then injected between the first and second major surfaces to form a compliant layer substantially surrounding the leads. The compliant layer has first and second major surfaces abutting the first and second sacrificial base sheets, respectively. The first and second sacrificial base sheets are then removed, leaving the first and second terminals affixed to the compliant layer at the first and second major surfaces, respectively.
The base sheets may be removed by etching them with an etchant that does not dissolve the terminals, thus leaving the terminals affixed to the compliant layer. In that case, the base sheets are provided with terminals that are resistant to the etchant used to dissolve the base sheets.
The step of bonding the second ends of the leads to the second terminals may include applying a conductive epoxy on the second ends of the leads or on the second terminals, joining the leads and terminals, and curing the epoxy.
According to a particularly preferred variant of this approach, the step of forming the leads includes bonding an end of a wire to one of the first terminals, severing the wire at a pre-determined distance from the first terminal, and repeating the forming and severing steps for each of the first terminals. The wires may be severed using a flame, which forms an enlarged end on the remaining wire suitable for bonding during a subsequent bonding operation, and an enlarged end on the formed lead suitable for bonding to a second terminal.
The method preferably includes the step of displacing the first and second sacrificial base sheets toward each other, thereby deforming the leads. The deformed leads provide resilience to the microelectronic connector, permitting relative movement between the ends of the leads in all directions.
In another method according to the invention, a lead bonding tool is used to form the leads. The lead bonding tool has a tool surface, a pattern of holes opening on the tool surface at locations corresponding to locations of first terminal portions of a first base sheet, and a plurality of lead wires having first ends protruding from the holes. The lead wires may be fed through the holes from a lead wire supply such as a coil. The base sheet and the lead bonding tool are brought together so as to bring the first ends of the lead wires into proximity with the terminal portions of the base sheet. The first ends of the lead wires are then bonded to the terminal portions using thermosonic bonding or other techniques.
The lead bonding tool is then moved away from the sacrificial base sheet so that the lead wires pay out from the lead bonding tool. A flowable material is then injected between the tool surface of the bonding tool and the first major surface of the base sheet to form a compliant layer that substantially surrounds the lead wires. The compliant layer has a first major surface abutting the first major surface of the base sheet, and a second major surface abutting the surface of the bonding tool. The bonding tool is then moved away from the compliant layer so that the lead wires pay out additionally from the lead bonding tool. In this step, the tool surface of the bonding tool is separated from the second major surface of the compliant layer.
The lead wires are then severed at or above the second major surface of the compliant layer, to form leads extending through the compliant layer. Surplus portions of the first sacrificial base sheet are then removed, leaving the first terminal portions affixed to the compliant layer at the first major surface of the compliant layer.
Preferably, the surplus portions of the first sacrificial base sheet are removed by etching. The etching agent dissolves the surplus portions of the first sacrificial base sheet while leaving the first conductive terminal portions substantially intact. The method may also include the step of bending the lead wires before they are severed.
In a most preferred version of this method, the step of severing the lead wires forms second ends of the leads extending from the second major surface of the compliant layer. The second ends of the leads are then etched to a point substantially flush with the second major surface of the compliant layer. A second sacrificial base sheet having a second major surface and a plurality of second conductive terminals disposed on the second major surface is then bonded to the connector. Specifically, the second ends of the leads are bonded to corresponding ones of the second terminals. The second sacrificial base sheet is then removed, leaving the second terminals affixed to the second major surface of the compliant layer.
The second ends of the leads may be bonded to corresponding second terminals using a conductive epoxy. Further, the step of severing the lead wires may comprise severing the wires with heat such as a flame so as to form enlarged ends of the leads. The enlarged ends protrude from the holes in the bonding tool.
The step of severing the lead wires may comprise severing the leads at a distance from the bonding tool sufficiently small to maintain substantial control of locations of the first ends of the lead wires during a subsequent lead wire bonding step. Thus, the lead wires extending from the bonding tool after the process is complete are ready for a subsequent performance of the steps of the process.
In another method of the invention, a first sacrificial base sheet is provided with first conductive terminals exposed on a first surface thereof. The base sheet further comprises recesses on the first major surface surrounding the first conductive terminals. A plurality of leads is then formed on the first conductive terminals, with first ends of the leads connected to the first terminals and second ends remote from the first base sheet. A compliant layer is then formed substantially surrounding the leads. The compliant layer has a first major surface abutting the first major surface of the first base sheet. The first sacrificial base sheet is then removed, exposing the first major surface of the compliant layer. The first conductive terminals remain affixed to the compliant layer on the first major surface of the compliant layer. The first major surface of the compliant layer has raised portions formed by the recesses in the sacrificial base sheet. The raised portions substantially surround the terminals.
The method may further include providing a facing element having a second major surface facing the first major surface of the sacrificial base sheet. The sacrificial base sheet and the facing element form a gap between their respective surfaces. The second major surface of the facing element has raised portions in locations corresponding to the recesses in the first major surface of the sacrificial base sheet. The compliant layer is formed by injecting a compliant material into the gap. The compliant layer then has a second major surface abutting the second major surface of the facing element and having recesses formed by the raised portions of the facing element. The recesses and the raised portions formed on the compliant layer increase the compliancy thereof.
The facing element may be a second sacrificial base sheet having a plurality of second conductive terminals on the second major surface between the raised portions. In this case, the method further includes the step of bonding the second ends of the leads to corresponding ones of the second terminals.
The facing element may alternatively be a lead bonding tool having a plurality of holes opening on the second major surface. A plurality of the lead wires in the bonding tool have first ends protruding from the holes. In this embodiment, the step of forming the leads also includes approaching the sacrificial base sheet with the lead bonding tool so as to bring the first ends of the lead wires into proximity with the first conductive terminals, bonding the first ends of the lead wires to the terminals, and moving the lead bonding tool away from the first sacrificial base sheet so that the lead wires pay out from the lead bonding tool. After the compliant layer is formed, the lead bonding tool is moved away from the compliant layer so that the lead wires pay out from the lead bonding tool. The leads are then severed at or above the second major surface of the compliant layer to form the leads.
This version of the method of the invention may also include the step of forming holes in the compliant layer extending substantially from the first major surface to the second major surface. The holes increase the compliancy of the compliant layer. The method may also include the step of etching the first major surface of the first base sheet around the conductive terminals so as to form the recesses in the base sheet.
These and other objects, features and advantages of the present invention will become more readily apparent from the detailed description of the preferred embodiment set forth below, taken in conjunction with the accompanying drawings.